This invention relates to a method of and apparatus for separation of radioactive krypton from a feed gas mixture comprising nitrogen and trace amounts of carbon dioxide and radioactive krypton by first selective adsorption and then cyrogenic distillation to produce highly concentrated radioactive gas and permit release of the remaining gas to the atmosphere without radioactive contamination.
In the normal operation of nuclear power plants of the boiling water reactor type, an off-gas stream is produced which is radioactive. Such streams threaten the environment primarily due to their krypton and xenon content, other significant radioactive components being short half-life isotopes which can be satisfactorily deactivated by storage prior to being released to the atmosphere. More specifically, the nuclear reactor off-gas contains radioactive isotopes of other gaseous elements such as oxygen, nitrogen and argon. The half-lives of these radioactive components vary widely from a fraction of a minute to about 10 minutes. For example, if a value of 1.0 were assigned to the radioactivity level of a quantity of off-gas at the moment it is withdrawn from the conventional vacuum condenser, its radioactive level would drop to about 1/90 and 1/550 after time lapses of 2 minutes and 30 minutes respectively. However, the residual radioactivity level after 30 minutes is still far greater than can be safely permitted if the gas is to be released to the atmosphere.
The discharge of radioactive off-gas creates both a short term and a long term problem, the former being the total radioactivity of the gas discharged to the atmosphere. Because the total radioactivity is produced largely by short half-life isotopes, the short term problem is localized within the immediate vicinity of the nuclear reactor where the radioactivity is not yet dispersed and decayed to negligble levels. The long term problem is concerned primarily with krypton-85 which has a half-life of greater than 10 years. Krypton-85 should be substantially removed from the off-gas to minimize the long term accumulation of radioactive contaminants throughout the earth's atmosphere.
In some instances, nuclear power plants have merely diluted the off-gas with additional volumes of air before venting to the atmosphere but this is no longer desirable. The only safe method to dispose the radioactive gas is to store it with adequate shielding for a time sufficient for the radioactivity to decay to prescribed tolerable levels, but the most recent stringent standards may require many weeks storage. The quantity of off-gas produced in a boiling water type nuclear power plant is quite large. For example, in a 1100 megawatt plant, the non-condensables comprising the off-gas may accumulate at the rate of 200-300 cfm. (STP), so that if this gas is held in several week's "decay-storage" the amount being retained in any given moment can readily be several million cubic feet (STP) requiring extremely large storage tanks which must be shielded to contain its radioactive emission.
One prior art approach has been to remove a part of the non-radioactive constituents of the off-gas so as to concentrate the radioactive portion to perhaps 25% of the original volume. This approach does not mitigate the long term storage decay problem created by krypton-85. Other nuclear power plants have delayed the release of the radioactive off-gas for a short period, e.g. one-half hour, by passing same through an extended pipe line system preceding the vent point. This system has been further improved at certain locations by inserting a large carbon adsorption delay trap in the extended pipe line system preceding the vent point. No means have been provided for cyclic adsorption and desorption of the adsorption trap, but by continuous adsorption displacement it retains the radioactive components for a longer period of time than obtainable in the pipe line alone and permits further decay of their radioactivity. In this manner, the krypton retention may be increased to several days but even this improvement fails to meet the increasing stringent limitations on total radioactivity level established by government agencies in many locations.
It is an object of this invention to provide an improved method of and apparatus for separating radioactive krypton from a gas mixture comprising nitrogen and trace amounts of carbon dioxide and the radioactive krytpon.
Another object is to provide an improved method of and apparatus for radioactive krypton removal from nuclear reactor off-gas so that the remaining gas may be released to the atmosphere without radioactive contamination.
Still another object is to provide such a system requiring only relatively small volume for storage-radioactive decay of the separated krypton.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and claims.